Diffraction type light-condensing film and planar light source device using the same

Abstract

Provided are an easy-to-handle thin diffraction type light-condensing film exhibiting high light transmissivity and condensation ability, and a planar light source device using the film. A hologram optical element using diffraction / interference phenomena based on wave properties of light is used instead of a conventional prism sheet using refraction. As a result, the diffraction type light-condensing film and the planar light source device have high light transmissivity and are thin. In the diffraction type light-condensing film, dependence of bending angle on wave length is low and light entering from an oblique direction is bent in the vertical direction and emitted with spectral separation of white light suppressed. High light-condensation impossible in a conventional optical element is realized by suppressing angular variation in emission light for angular variation in incident light.

Description

TECHNICAL FIELD [0001] The present invention relates to a diffraction type light-condensing film for bending oblique incident white light to a vertical direction and outputting the bent light and a planar light source device using the same. In particular, the present invention is applicable to a backlight of a liquid crystal display, to improve the brightness of the display in the direction of an observer. BACKGROUND TECHNOLOGY [0002] Liquid crystal displays (LCDs) are used as display parts of computers, display parts of control panels of home appliances, and display parts of cellular phones and are required to further reduce power consumption, weight, and thickness. [0003] The liquid crystal display is not a self-luminous device, and therefore, must have an external light source or environmental external light. The typical external light source is a backlight, which is a planar light source placed behind an LCD panel. The backlight is required to emit the outgoing light in the dire...

AI Technical Summary

Benefits of technology

[0010] There is provided a diffraction type light-condensing film that is thin, has a high light transmittance and light-condensing ability, and is easy to handle, as well as a planar light source device using the same.

[0012] A diffraction type light-condensing film according to the present invention bends white light with oblique incident angle in the normal direction of the firm surface, has small wavelength dependence of the bending angle, suppresses the dispersion of the light, and emits the light as white light. At the same time, the film minimizes a change in the angle of the output light with respect to a change in the angle of incident light, to realize a high light-condensing ability that is unachievable with a conventional optical element.

[0020] It is preferable that a film having a function of preventing polarization split, color separation, or reflection is arranged adjacent to the transmission diffraction grating, or on the front and back sides of a hologram optical element.

[0021] It is preferable that the function of preventing polarization split, color separation, or reflection is provided by a relief grating having a period equal to or smaller than 0.6 μm and a depth equal to or smaller than 0.5 μm.

Problems solved by technology

In the vicinity of the light-entering part of a side-incidence backlight, the uniformity of the brightness is so inferior as to deteriorate the quality of the image displayed on the LCD panel placed over the backlight.

Due to the low uniformity, the region within a certain distance from the light-entering part must be reserved as an undisplayed area, which hinders the miniaturization of LCD's.

It is difficult for conventional prism sheets to simultaneously realize high brightness and high brightness uniformity.

The dispersion deteriorates the color reproduction of liquid crystals to degrade display quality.

With the macroscopic geometry, geometric-optical sheets, including the prism sheet, cannot help remaining thick.

Thus, the prism sheet is an obstacle in the attempt to make thinner backlight.

As each prism in the prism sheet independently functions to bend light, and defect or foreign matter in the prism results in an anomalous scattering of the light that passes through the prism.

A display is sensitive to the defect or foreign matter and causes a display abnormity, to deteriorate product quality.

Such a hologram optical element using the diffraction / interference phenomenon based on wave optics has problems of 1) producing diffracted light of orders of diffraction other than that at which incident light is vertically diffracted, 2) lowering the diffraction efficiency of the required order of diffraction, and 3) causing large wavelength dispersion.

If a depth is improper, the diffraction efficiency of the required order of diffraction will be low.

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